Capacitance to frequency converter

ABSTRACT

A capacitance to frequency converter includes a switching capacitor circuit, a charge dissipation circuit, a comparator, and a signal generator. The switching capacitor circuit charges a sensing capacitor and transfers charge from the sensing capacitor to a circuit node of the charge dissipation circuit. The comparator is coupled to the charge dissipation circuit to compare a potential at the circuit node to a reference voltage. The signal generator is coupled to an output of the comparator and to the charge dissipation circuit. The signal generator is responsive to the output of the comparator to generate a signal fed back to control the charge dissipation circuit. A frequency of the signal is proportional to a capacitance of the sensing capacitor.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/947,871, filed on Jul. 3, 2007, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to electronic circuits, and in particular but not exclusively, relates to capacitance measurement circuits.

BACKGROUND INFORMATION

Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. In general, capacitive sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. A capacitive sensor permits eliminating complicated mechanical switches and buttons, providing reliable operation under harsh conditions. Capacitive sensors are widely used in the modern consumer applications, providing new user interface options in the exiting products (cell phones, digital music players, personal digital assistances, etc.).

One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1A, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed). Switches SW1 and SW2 are operated in a non-overlapping manner repeating the transfer of charge from Cx to Csum.

Capacitance sensor 100 is operated to measure the capacitance of Cx in the following manner. In an initial stage, Csum is reset by discharging Csum by temporarily closing switch SW3. Then, switches SW1 and SW2 commence operating in the two non-overlapping phases that charge Cx and transfer the charge from Cx into Csum. The voltage potential on Csum rises with each charge transfer phase, as illustrated in FIG. 1B. The voltage on Csum can by calculated according to equation 1.

$\begin{matrix} {V_{Csum} = {V_{dd}\left( {1 - {\mathbb{e}}^{{- N}\frac{Cx}{Csum}}} \right)}} & \left( {{Equation}\mspace{14mu} 1} \right) \end{matrix}$ where V_(Csum) represents the voltage on Csum, N represents the cycle count, Cx and Csum represent capacitance values, and Vdd represents a power supply voltage. Accordingly, the capacitance of Cx can be determined by measuring the number of cycles (or time) required to raise Csum to a predetermined voltage potential.

The charge transfer method is advantageous due to its relative low sensitivity to RF fields and RF noise. This relative noise immunity stems from the fact that the sensing capacitor Cx is typically charged by a low-impedance source and the charge is transferred to a low-impedance accumulator (i.e., the summing capacitor Csum). However, conventional capacitance sensors have the disadvantage that that voltage on the summing capacitor Csum rises versus time/cycles in an exponential manner (see FIG. 1B and Equation 1). The exponential relationship between the accumulated voltage potential on Csum and the charge transfer time/cycles requires some linearization if the capacitance of Cx is calculated as a function of the voltage potential on Csum after a predetermined time or number of cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1A is a circuit diagram illustrating a conventional capacitance sensor circuit.

FIG. 1B is a graph illustrating the exponential relationship between voltage on a summing capacitor and charge transfer cycles.

FIG. 2 is a circuit diagram illustrating a capacitance to frequency converter, in accordance with an embodiment of the invention.

FIG. 3 is a flow chart illustrating a process of operation of a capacitance to frequency converter, in accordance with an embodiment of the invention.

FIG. 4 is a circuit diagram illustrating the equivalent circuit resistance of a switching capacitor circuit, in accordance with an embodiment of the invention.

FIG. 5 is a line graph illustrating the output of a pulse generator having a zero level fixed length pulse during an active mode of operation, in accordance with an embodiment of the invention.

FIG. 6 includes two line graphs illustrating the relationship between a MOD_EN signal generated by a comparator and a feedback pulse signal generated by a pulse generator, in accordance with an embodiment of the invention.

FIGS. 7A-C are circuit diagrams illustrating alternative dissipation circuit implementations, in accordance with embodiments of the invention.

FIG. 8 is a functional block diagram illustrating a demonstrative processing system for implementing a capacitive sense user interface using a capacitance to frequency converter, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of an apparatus and method of operation for a capacitance to frequency converter are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 2 is a circuit diagram illustrating a capacitance to frequency converter 200, in accordance with an embodiment of the invention. Capacitance to frequency converter 200 is capable of converting the measurement of the capacitance of sensing capacitor (Cx) into the measurement of the frequency of a feedback pulse signal (FB). Furthermore, the relationship between the frequency of FB (f_(FB)) and the capacitance of Cx is substantially linear. However, it should be appreciated that capacitance to frequency converter 200 may also be used for other functions such as a voltage to frequency converter or current to the frequency converter. Although the system of FIG. 2 is referred to as a capacitance to “frequency” converter, it should be appreciated that the system of FIG. 2 may also be referred to as a capacitance to period converter, since period is merely the reciprocal of frequency.

The illustrated embodiment of capacitance to frequency converter 200 includes a switching capacitor circuit 205, a charge dissipation circuit 207, an analog comparator (“CMP”) 210, a pulse generator 215 with enable input, and a frequency measurement circuit 220. The illustrated embodiment of switching capacitor circuit 205 includes sensing capacitor (Cx), a charging switch (SW1), a charge transfer switch (SW2), and a clocking circuit 209. The illustrated embodiment of charge dissipation circuit 207 includes a modulation capacitor (Cmod), a discharge resistor (Rd), and a discharge switch (SW3). While component values of switching capacitor circuit 205 may vary based on the particular application, in general, the capacitance of Cmod will be substantially larger than the capacitance of Cx. Since Cmod acts to accumulate charge transferred from Cx over multiple cycles, it is often referred to as a summing capacitor or an integrating capacitor. Pulse generator 215 generates fixed length pulses synchronized to MOD_EN output from CMP 210. Pulse generator 215 may also be referred to as a sync generator due to the sync waveform it generates.

FIG. 3 is a flow chart illustrating a process 300 for operating capacitance to frequency converter 200, in accordance with an embodiment of the invention. The order in which some or all of the process blocks appear in each process should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel.

In a process block 305, capacitance to frequency converter 200 is powered on and the output of comparator 210 (MOD_EN) is initially low, assuming Cmod is initially discharged and the voltage Umod on the non-inverting input of CMP 210 is less than Vref on its inverting input. In this state, pulse generator 215 is “disabled” generating a constant logic value for the feedback signal FB, which open circuits switch SW3 (process block 310).

With power provided to the circuit, switches SW1 and SW2 also commence operation. Switches SW1 and SW2 switch, under control of a clock signal CLK distributed by clocking circuit 209 as two non-overlapping phases Phi1 and Phi2, to reciprocally charge Cx and then transfer the charge from Cx onto Cmod (process block 315). The non-overlapping charge and charge transfer phases may be sufficiently spaced to prevent cross conduction or latch up between SW1 and SW2. During operation, switching capacitor circuit 205 has an equivalent resistance Rc given by equation 2.

$\begin{matrix} {R_{C} = \frac{1}{f_{CLK}C_{x}}} & \left( {{Equation}\mspace{14mu} 2} \right) \end{matrix}$ where f_(CLK) represents the frequency of the clock signal CLK and Cx represents the capacitance of the sensing capacitor Cx. FIG. 4 illustrates the circuit equivalent resistance Rc. The clock signal CLK may be a fixed frequency signal, a spread spectrum clock signal, or otherwise.

As switching capacitor circuit 205 begins charging Cmod, the voltage potential Umod at node N1 begins to rise gradually with each charge transfer through switch SW2. Cmod continues to accumulate charge transferred from Cx until Umod reaches Vref, as determined by CMP 210 (decision block 320). When Umod reaches or passes Vref, CMP 210 toggles its output (MOD_EN) to enable pulse generator 215 (process block 325).

The illustrated embodiment of pulse generator 215 produces the FB signal having fixed width negative pulses (e.g., active is logic zero level). However, it should be appreciated that pulse generator 215 may also be implemented as an active high sync generator for generating fixed length positive pulses. With reference to FIG. 5, the duration of the zero level pulses are always fixed at a value t₀. Since pulse generator 215 outputs an active low signal, the duty cycle of FB of the illustrated embodiment of pulse generator 215 is defined according to equation 3.

$\begin{matrix} {{{Duty}\mspace{14mu}{Cycle}} = \frac{t_{0}}{T}} & \left( {{Equation}\mspace{14mu} 3} \right) \end{matrix}$ where T is the period of the FB signal and t_(o) represents the duration of the zero level pulses.

Pulse generator 215 operates in one of three operation modes: 1) when the MOD_EN signal is active constantly, FB is switched with a minimal fixed period having a fixed duty cycle, 2) when the MOD_EN signal is switched from the active state (e.g., ON) to the inactive state (e.g., OFF), FB returns to the inactive high state only after completing the current zero level pulse, and 3) while the MOD_EN signal is in the inactive state (e.g., OFF), FB is output as a static logical high. FIG. 6 illustrates how the active state of the MOD_EN signal generates zero level pulses from pulse generator 215. When the MOD_EN signal transitions from the active state to the inactive state, the current zero level pulse is completed, as illustrated by arrow 605.

As FB continues to toggle while the MOD_EN signal is in the active state, switch SW3 toggles between open and closed states discharges Cmod through discharge resistor Rd (process block 335). Cmod continues to periodically discharge through Rd until Umod drops below Vref (decision block 340). At this point, CMP 210 toggles the MOD_EN signal, deactivating pulse generator 215 (process block 310) and returning FB to the inactive state. With FB returned to the high level inactive state, switch SW3 is open circuited and the charging of Cmod repeats in process block 315.

After an initial transitory startup phase, capacitance to frequency converter 200 enters its steady state phase where the voltage potential Umod on Cmod oscillates or dithers about Vref. This oscillation about Vref creates the modulation signal MOD_EN used to enable/disable pulse generator 215 to thereby modulate the frequency of the feedback pulse signal FB. Once operating in the steady state phase, the frequency of the feedback pulse signal is directly proportional to the capacitance of Cx as seen by equations 4, 5, and 6 below. The current I_(Cx) through the equivalent resistance Rc is

$\begin{matrix} {I_{Cx} = {\frac{{Vdd} - {Vref}}{Rc} = {\left( {{Vdd} - {Vref}} \right) \cdot {f_{{CLK}^{Cx}}.}}}} & \left( {{Equation}\mspace{14mu} 4} \right) \end{matrix}$ Since the sum of currents into and out of node N1 must equal zero, the average current I_(Rd) through discharge resistor Rd is

$\begin{matrix} {{I_{RD} = {{\frac{Vref}{Rd} \cdot \frac{t_{0}}{T}} = {{{\frac{Vref}{Rd} \cdot t_{o}}F} = I_{cx}}}},} & \left( {{Equation}\mspace{14mu} 5} \right) \end{matrix}$ where F represents the frequency or 1/T of the FB signal. Finally, by rearranging equation 5 and plugging in equation 4 to eliminate the I_(Cx) variable,

$\begin{matrix} {F = {\left( {\frac{Vdd}{Vref} - 1} \right) \cdot \frac{f_{CLK}R_{d}}{t_{o}} \cdot {{Cx}.}}} & \left( {{Equation}\mspace{14mu} 6} \right) \end{matrix}$

As can be seen from equation 6, at fixed values for Vdd, Vref, Rd, and fclk, the dependence of F on Cx is linear. In short, pulse generator 215 is synchronized to MOD_EN and produces pulses FB with a frequency directly proportional to the capacitance of Cx. Accordingly, in a process block 345, frequency measurement circuit 220 measures the frequency of the feedback pulse signal FB. The measured frequency values can then be used to determine the capacitance Cx or capacitance change ΔCx of the sensing capacitor (process block 350). In one embodiment, frequency measurement circuit 220 may include a simple counter to measure the frequency of the feedback pulse signal FB. For fixed values of Vdd, Rd, and Cx, capacitance to frequency converter 200 operates as a voltage (Vref) to frequency converter where the output frequency of FB is selected by appropriate selection of Vref.

FIGS. 7A-C are circuit diagrams illustrating alternative implementations of charge dissipation circuit 207, in accordance with embodiments of the invention. FIG. 7A illustrates a charge dissipation circuit 705, which replaces SW3 and discharge resistor Rd with a current source 707 controlled by feedback pulse signal FB. When FB is a logic HIGH, current source 707 sinks a current I_(D) from modulation capacitor Cmod to ground. When FB is logic LOW, current source 707 is disabled.

FIG. 7B illustrates a charge dissipation circuit 710 including a switching capacitor resistor circuit 712 with a gated clock source. When FB is logic HIGH, the clock signal CLK is applied to the switches SW4 and SW5 with non-overlapping pulses (e.g., such as clock signals Phi1 and Phi2), causing a discharging current to flow to ground from modulator capacitor Cmod. At a logic LOW value for FB, the clock signal CLK is gated and switching capacitor circuit 712 does not sink current from modulator capacitor Cmod.

FIG. 7C illustrates a charge dissipation circuit 715 where the non-overlapping clock phases Phi1 and Phi2 are applied constantly to switches SW4 and SW5, but SW4 and SW5 are selectively connected in series between Umod and either Vref or ground by the multiplexor MUX, depending on the value of the feedback pulse signal FB. The principle of operating of charge dissipation circuit 715 is similar to charge dissipation circuit 710 in that SW4, SW5, and Ccomp operate as a switching capacitor resistor circuit.

FIG. 8 is a functional block diagram illustrating a demonstrative system 1100 for implementing a capacitive sense user interface, in accordance with an embodiment of the invention. The illustrated embodiment of system 1100 includes a processing device 1110, a capacitive sense pad 1120, a capacitive sense linear slider 1130, a capacitive sense radial slider 1140, a host processor 1150, an embedded controller 1160, and non-capacitance sensor elements 1170. Processing device 1110 may include analog and/or digital general purpose input/output (“GPIO”) ports 1107. GPIO ports 1107 may be programmable. GPIO ports 1107 may be coupled to a Programmable Interconnect and Logic (“PIL”), which acts as an interconnect between GPIO ports 1107 and a digital block array of processing device 1110 (not illustrated). The digital block array may be configured to implement a variety of digital logic circuits (e.g., DAC, digital filters, digital control systems, etc.) using, in one embodiment, configurable user modules (“UMs”). The digital block array may be coupled to a system bus. Processing device 1110 may also include memory, such as random access memory (RAM) 1105 and program flash 1104. RAM 1105 may be static RAM (“SRAM”), and program flash 1104 may be a non-volatile storage, which may be used to store firmware. Processing device 1110 may also include a memory controller unit (“MCU”) 1103 coupled to memory and the processing core 1102.

Processing device 1110 may also include an analog block array (not illustrated). The analog block array is also coupled to the system bus. The analog block array also may be configured to implement a variety of analog circuits (e.g., ADC, analog filters, etc.) using, in one embodiment, configurable UMs. The analog block array may also be coupled to the GPIO 1107.

As illustrated, capacitance sensor 1101, which includes an implementation of capacitance to frequency converter 200 may be integrated into processing device 1110. Capacitance sensor 1101 may include analog I/O for coupling to an external component, such as capacitive sense pad 1120, capacitive sense linear slider 1130, capacitive sense radial slider 1140, and/or other capacitive sense devices. Capacitive sense pad 1120, capacitive sense linear slider 1130, and/or capacitive sense radial slider 1140 may each include one or more sensing capacitors Cx to implement the individual capacitive sense buttons therein.

Processing device 1110 may include internal oscillator/clocks 1106 and communication block 1108. The oscillator/clocks block 1106 provides clock signals to one or more of the components of processing device 1110. Communication block 1108 may be used to communicate with an external component, such as a host processor 1150, via host interface (I/F) line 1151. Alternatively, processing device 1110 may also be coupled to embedded controller 1160 to communicate with the external components, such as host 1150. Interfacing to the host 1150 can be through various methods. In one exemplary embodiment, interfacing with the host 1150 may be done using a standard PS/2 interface to connect to embedded controller 1160, which in turn sends data to the host 1150 via low pin count (LPC) interface. In some instances, it may be beneficial for processing device 1110 to do both touch-sensor pad and keyboard control operations, thereby freeing up the embedded controller 1160 for other housekeeping functions. In another exemplary embodiment, interfacing may be done using a universal serial bus (USB) interface directly coupled to host 1150 via host interface line 1151. Alternatively, processing device 1110 may communicate to external components, such as host 1150 using industry standard interfaces, such as USB, PS/2, inter-integrated circuit (I2C) bus, or system packet interfaces (SPI). Host 1150 and/or embedded controller 1160 may be coupled to processing device 1110 with a ribbon or flex cable from an assembly, which houses the sensing device and processing device.

In one embodiment, processing device 1110 is configured to communicate with embedded controller 1160 or host 1150 to send and/or receive data. The data may be a command or alternatively a signal. In an exemplary embodiment, system 1100 may operate in both standard-mouse compatible and enhanced modes. The standard-mouse compatible mode utilizes the HID class drivers already built into the Operating System (OS) software of host 1150. These drivers enable processing device 1110 and sensing device to operate as a standard cursor control user interface device, such as a two-button PS/2 mouse. The enhanced mode may enable additional features such as scrolling (reporting absolute position) or disabling the sensing device, such as when a mouse is plugged into the notebook. Alternatively, processing device 1110 may be configured to communicate with embedded controller 1160 or host 1150, using non-OS drivers, such as dedicated touch-sensor pad drivers, or other drivers known by those of ordinary skill in the art.

Processing device 1110 may reside on a common carrier substrate such as, for example, an integrated circuit (IC) die substrate, a multi-chip module substrate, or the like. Alternatively, the components of processing device 1110 may be one or more separate integrated circuits and/or discrete components. In one exemplary embodiment, processing device 1110 may be a Programmable System on a Chip (PSoC™) processing device, manufactured by Cypress Semiconductor Corporation, San Jose, Calif. Alternatively, processing device 1110 may be one or more other processing devices known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”), or the like. In an alternative embodiment, for example, processing device 1110 may be a network processor having multiple processors including a core unit and multiple microengines. Additionally, processing device 1110 may include any combination of general-purpose processing device(s) and special-purpose processing device(s).

Capacitance sensor 1101 may be integrated into the IC of processing device 1110, or alternatively, in a separate IC. Descriptions of capacitance sensor 1101 may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code describing capacitance sensor 1101, or portions thereof, may be generated using a hardware descriptive language, such as VHDL or Verilog, and stored to a machine-accessible medium (e.g., CD-ROM, hard disk, floppy disk, etc.). Furthermore, the behavioral level code can be compiled into register transfer level (“RTL”) code, a netlist, or even a circuit layout and stored to a machine-accessible medium. The behavioral level code, the RTL code, the netlist, and the circuit layout all represent various levels of abstraction to describe capacitance sensor 1101.

In one embodiment, electronic system 1100 may be used in a notebook computer. Alternatively, system 1100 may be used in other applications, such as a mobile handset, a personal data assistant (PDA), a keyboard, a television, a remote control, a monitor, a handheld multi-media device, a handheld video player, a handheld gaming device, or a control panel.

The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a machine (e.g., computer) readable medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or the like.

A machine-accessible medium includes any mechanism that provides (e.g., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-accessible medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.).

The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. 

1. A capacitance to frequency converter, comprising: a switching capacitor circuit to charge a sensing capacitor and to transfer charge from the sensing capacitor to a charge dissipation circuit via a circuit node; a comparator coupled to the charge dissipation circuit to compare a potential at the circuit node to a reference voltage; and a signal generator coupled to an output of the comparator and to the charge dissipation circuit, the signal generator responsive to the output of the comparator to generate a signal fed back to control the charge dissipation circuit, wherein a frequency of the signal is proportional to a capacitance of the sensing capacitor, and wherein the charge dissipation circuit comprises a modulation capacitor coupled to the circuit node and a current source coupled to selectively discharge the modulation capacitor in response to the signal.
 2. The capacitance to frequency converter of claim 1, wherein the signal generator comprises a pulse generator and the signal comprises a pulse signal.
 3. The capacitance to frequency converter of claim 2, wherein the pulse generator generates a zero level fixed length pulse having a period between adjacent fixed length pulses modulated by the output of the comparator.
 4. The capacitance to frequency converter of claim 2, further comprising a frequency measurement circuit coupled to the pulse generator to measure a frequency of the pulse signal.
 5. The capacitance to frequency converter of claim 1, wherein the charge dissipation circuit comprises: a modulation capacitor coupled to the circuit node; a charge dissipation resistor; and a discharging switch coupled in series with the charge dissipation resistor to selectively discharge the modulation capacitor through the charge dissipation resistor in response to the signal.
 6. The capacitance to frequency converter of claim 5, wherein the switching capacitor circuit further comprises: a charging switch coupled to selectively charge the sensing capacitor; and a charge transfer switch coupled between the sensing capacitor and the modulation capacitor to selectively transfer charge from the sensing capacitor to the modulation capacitor.
 7. The capacitance to frequency converter of claim 6, further comprising a clocking circuit coupled to operate the charging switch and the charge transfer switch in two non-overlapping phases.
 8. The capacitance to frequency converter of claim 1, wherein the comparator comprises an analog comparator.
 9. The capacitance to frequency converter of claim 1, wherein the charge dissipation circuit comprises: a modulation capacitor coupled to the circuit node; a switching capacitor resistor circuit coupled between the circuit node and a ground; and a clock source gated by the signal coupled to control the switching capacitor resistor circuit.
 10. The capacitance to frequency converter of claim 1, wherein the charge dissipation circuit comprises: a modulation capacitor coupled to the circuit node; and a switching capacitor resistor circuit coupled to the circuit node and having two switches selectively coupled in series between the circuit node and either a ground or the reference voltage dependent upon the signal.
 11. A method of converting a capacitance value to a frequency value, the method comprising: reciprocally switching charge into a first capacitor having the capacitance value and out of the first capacitor into a second capacitor; selectively modulating a frequency of a pulse signal in response to the charge accumulated on the second capacitor; and selectively discharging the charge from the second capacitor in response to the pulse signal.
 12. The method of claim 11, wherein selectively modulating the frequency of the pulse signal comprises: comparing a voltage potential on the second capacitor to a reference voltage potential; generating a modulation signal dependent upon whether the voltage potential is greater or less than the reference voltage; and modulating the frequency of the pulse signal based on the modulation signal.
 13. The method of claim 12, wherein modulating the frequency of the pulse signal comprises modulating a period between fixed length pulses.
 14. The method of claim 13, wherein modulating the frequency of the pulse signal further comprises: selectively enabling a pulse generator in response to the modulation signal; generating the pulse signal with a minimal fixed period when the modulation signal is in a first state; and generating the pulse signal having a fixed logic level between the fixed length pulses while the modulation signal is in a second state.
 15. The method of claim 14, wherein modulating the period between the fixed length pulses comprises completing generation of a current one of the fixed length pulses prior to reverting to the fixed logic level after the modulation signal transitions to the second state.
 16. The method of claim 11, wherein reciprocally switching charge into the first capacitor having the capacitance value and out of the first capacitor into the second capacitor comprises two non-overlapping switching phases.
 17. An apparatus, comprising: a switching capacitor circuit to charge a first capacitor and to transfer charge between the first capacitor and a modulation capacitor; a comparator coupled to the modulation capacitor to compare a potential on the modulation capacitor to a first voltage and to generate a modulation signal; a pulse generator coupled to the comparator to receive the modulation signal and to generate a feedback signal responsive to the modulation signal; and a discharging circuit coupled to selectively discharge the modulation capacitor responsive to the feedback signal.
 18. The apparatus of claim 17, wherein an enable input of the pulse generator is coupled to the modulation signal generated by the comparator.
 19. The apparatus of claim 17, wherein the discharging circuit comprises: a charge dissipation resistor; and a discharging switch coupled in series with the charge dissipation resistor to selectively discharge the modulation capacitor through the charge dissipation resistor in response to the feedback signal.
 20. The apparatus of claim 17, further comprising a frequency measurement circuit coupled to the pulse generator to measure a frequency of the feedback signal, wherein the feedback signal comprises a pulse signal and wherein the frequency is proportional to a capacitance of the sensing capacitor.
 21. The apparatus of claim 17, wherein in the switching capacitor circuit further comprises: a charging switch coupled to selectively charge the sensing capacitor; and a charge transfer switch coupled between the sensing capacitor and the modulation capacitor to selectively transfer charge from the sensing capacitor to the modulation capacitor.
 22. The apparatus of claim 21, further comprising a clocking circuit coupled to operate the charging switch and the charge transfer switch in two non-overlapping phases. 